Currently, a ship with a gross tonnage greater than or equal to 20 tons is worldwide obligated to install a Global Maritime Distress and Safety System (GMDSS) as a radio communication system therein. However, a GMDSS is expensive, and hence, a fishing vessel with a gross tonnage less than 20 tons is released from installation of the GMDSS by a “deemed GMDSS”, where a report of positional information to a fishing industry radio association that controls a fishing vessel is orally executed three times a day by a fishing industry radio.
Patent Document 1: Japanese Laid-open Patent Publication No. 05-083180
Patent Document 2: Japanese Laid-open Patent Publication No. 2006-295397
Patent Document 3: Japanese Laid-open Patent Publication No. 07-203519
However, in the deemed GMDSS, a fishing vessel may be unable to respond in operation or no radio wave from a fishing industry radio may reach a fishing vessel depending on a propagation state of the radio wave. Because a fishing industry radio is provided for audio communication such as a Single Side Band (SSB) using, for example, a shortwave band, its reception state may greatly vary depending on a state of an ionospheric layer so as to cause the communication to be difficult and be unable to be in contact for a long period of time. Accordingly, for a fishing vessel that is unable to be in contact, it is impossible to determine whether no radio wave transmitted from a base station reaches the fishing vessel or an unexpected contingency occurs on the fishing vessel. In a case where communication is executed between land and a ship, as well as the deemed GMDSS, a case of being unable to be in contact for a long period of time may be provided similarly, and it is impossible to determine whether no radio wave transmitted from a base station reaches the ship or an unexpected contingency occurs on the ship.